Many diseases manifest themselves in a localized manner. These diseases include, for example, diseases of the breast, liver and prostate. Surgical procedures have traditionally been employed after medicinal approaches have been shown to be unsuitable or ineffective. Surgery, however, may still subject a patient to a significant risk and may not be completely effective.
Some non-invasive techniques have been utilized in the prior art for treating diseases of tissues, organs and the like. For instance, one known non-invasive treatment is high-intensity focused ultrasound (HIFU) to produce volume lesions at the site of the diseased tissue within the body. In HIFU, the acoustic energy applied to treat a location in the body results in the therapy.
The ultrasound systems of the prior art usually employ a transducer to apply the ultrasound waves (e.g., the therapy). The transducer is often larger, and radiates higher power, than those used for imaging and visualization in the prior art. Illustrative systems are described in U.S. Pat. Nos. 4,484,569 and 4,858,613. In some systems, a single transducer is used for visualization and therapy. For instance, in U.S. Pat. No. 5,117,832, entitled "Curved Rectangular/Elliptical Transducer", by Sanghvi et al., an ultrasound transducer is described that includes element(s) that provide imaging and elements that provide therapy.
As a general rule, the maximum total acoustic power (TAP) output from an ultrasound transducer is a function of the area of the radiating surface of the transducer. To generate the high power necessary for certain therapies, large radiating surfaces are needed. One problem associated with non-invasive therapies such as HIFU is that the high intensity ultrasonic waves used are capable of damaging the surface of the body where the waves enter it. For example, this may cause undesired damage to the skin. It is desirable to use a high acoustic power for therapy in such a way as to reduce the damaging affects on the body at the entrance location of the ultrasonic waves.
In order to treat a volume of tissue within the body, the focal zone must be moved along all three dimensions. This is accomplished in prior art by moving the transducer, both along a plane parallel to the surface of the body, as well as towards and away from the body. However, due to limitations of space around the area to be treated, in some cases the transducer cannot be moved closer to the body to achieve therapy into a deeper location. Also, moving the transducer away from the body in order to treat regions closer to the surface may not be practical because the ratio of dosage at the focal zone relative to the entrance surface diminishes, which may result in damage to the tissue at the surface. A common approach in the prior art is to use one transducer of a shorter focal distance to treat volumes closer to the surface, and another transducer of a longer focal distance to treat volumes farther from the surface. But changing transducers is time consuming and, in some cases, uncomfortable to the patient.
For these reasons it is desirable to change the depth of the focal zone of a transducer without moving the transducer towards or away from the body.
The present invention provides a transducer for use in a treatment system. The transducer of the present invention is able to vary the depth of focus. The present invention is able to vary the location of the focus without varying the acoustic energy received at the different depths. The present invention is able to vary the focus while maintaining a high ratio between the intensity at the focal zone in relation to the intensity at the entrance surface.